This invention relates to an exposure device for copying machines, and more particularly to such an exposure device for use in an electrophotographic copying machine in which an original document to be reproduced is placed upon a transparent support platen with the original being scanned from below a moving light source for effecting exposure by reflecting light rays.
Electrophotographic copying machines, generally, consist of a cylindrical metallic light-sensitive member coated on its surface with a photosensitive or photoconductive material, a charging device for sensitizing the light sensitive member, an exposure device for dissipating in part the electrostatic charge placed upon the surface of the light system member to form a pattern of charge in conformity with the original image, a developing device for developing the charge pattern with a developing material including a resinous powder (to be referred to hereinafter as toner) capable of being attracted by the electrostatic attraction of the charge, a transfer device for transferring the toner image formed on the light sensitive member onto an image receptor such as paper, a fixing device for permanently fixing the toner image onto the image receptor, and a cleaning device for removing the excess toner remaining on the surface of a light sensitive member after transfer.
In copying machines having the above described construction, where a photo-positive original which is most frequently used is reproduced, the parts of charge corresponding to the characters and drawings remain after exposure on the surface of the light sensitive member, and other parts of charge corresponding to the background, i.e., the white areas of the original, are dissipated by the exposure. In the developing step, the charge remaining in the form of characters and drawings is developed with the toner charged and an opposite polarity to that of the latent image so as to form a toner image.
In copying machines that accommodate originals of various sizes, it is often convenient or desirable to effect copying without covering the support platen with the platen cover when the number of originals is large and a copy is required for each original or copying is to be effected quite hurriedly.
For example, when copying is effected with a platen cover 1 open as illustrated in FIG. 1 to make a copy of an original 2 which is smaller in size than the exposure area, the light rays projected from the exposure lamps onto the areas surrounding the original 2 will be reflected neither by the original nor by the platen cover 1 so that the charge 4 on the corresponding areas of the light sensitive member will remain undissipated. As a result, the remaining charge 4 will attract toner in development station 5. This is more conspicuous in a copying machine wherein an improved development system of high performance is used which is capable of reproducing with high fidelity half-tones and solid-block area photographs. In such an improved copying machine, a copy 6 with a black frame is obtained if the image receptor is larger than the original. On the other hand, where the dimension of the image receptor is substantially the same or smaller than that of the original, the toner 7 attracted to the parts of the light system member corresponding to the areas surround the original is not transferred onto the image receptor and will likely cause various undesirable phenomenon, that is, an increase in toner consumption, an increase in the load against a cleaning device 8, and smudging of machine parts and the like.
It is therefore an object of the present invention to overcome the above described disadvantages by providing an exposure device with light reflectors.
It is yet another object of the present invention to provide an exposure device for use in a copying machine with light reflectors at the ends of a support platen capable of being moved according to the sizes of copy paper being used so as to cover the ends of the support platen.